The Orthogonal Frequency Division Multiplexing (OFDM) technology is a multi-carrier communication technology, which distributes data to be transmitted to a plurality of sub-carriers to be transmitted in parallel. The intervals between the sub-carriers are equal to a baud rate and the sub-carriers are orthogonal to one another in the frequency domain. The transform from a frequency domain to a time domain and the transform from a time domain to a frequency domain are completed at the transmitting side and the receiving side, respectively, by using Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT). In order to eliminate Inter-Symbol Interference (ISI), a Cyclic Prefix (CP) is introduced in an OFDM system, and there is no interference between OFDM symbols when the length of the CP is larger than the maximum extension delay of a channel. The CP reserves the cycle characteristics of IFFT/FFT, and it is equivalent that each sub-carrier experiences a flat fading channel. Thus a frequency domain equalization can be realized by using an equalizer with a single tap, which is simpler than a multi-tap time domain equalization in a single-carrier system. As compared with a single-carrier signal, an OFDM signal has a spectrum more approximate to a rectangle, and its spectrum efficiency is higher since less bandwidth is occupied. The OFDM signal has another advantage that it can flexibly perform power distribution and format modulation for each sub-carrier, so that the OFDM signal is more suitable for a channel having complex frequency domain fading characteristics, thereby maximizing the capacity.
In the field of optical communications, the coherent optical communication can achieve better performance and higher spectrum efficiency than the traditional intensity modulation-direct detection optical communication system, and it is deemed as the main technology for realizing the next generation high-speed and large-capacity optical communication system. The OFDM technology widely used in the field of wireless communications can also be applied to the coherent optical communication, i.e., the Coherent Optical OFDM (CO-OFDM). In order to further improve the system capacity, two orthogonal polarization states of light may be used to transmit information in the CO-OFDM system, which is referred to as Dual Polarization (DP) CO-OFDM.
FIG. 1 schematically illustrates a functional block diagram of a conventional DP-CO-OFDM receiver. As illustrated in FIG. 1, in the conventional DP-CO-OFDM receiver, a received signal with two polarization states rh(t) and rv(t) is firstly transformed into a signal in a frequency domain, after undergoing a symbol synchronization process by a symbol synchronization unit 101, a carrier synchronization process by a carrier synchronization unit 102 and an FFT by an FFT unit 103. The obtained signal in the frequency domain goes through a channel estimation and equalization by a channel estimation and equalization unit 104 and, then, experiences a phase recovery in a phase recovery unit 105 and a data recovery in a data recovery unit 106, thereby completing the reception of the signal transmitted by a transmitter.
During the study of the present invention, the inventor of the present invention studies the receivers of the relevant art, and finds that a nonlinear effect of an optical fiber is a main limiting factor for the coherent optical OFDM system in the relevant art, and the detailed analysis is given as follows.
A traditional channel estimation and equalization method is based on training data. The conventional channel estimation and equalization unit 104 estimates one 2×2 channel inverse matrix for each sub-carrier, and multiplies it with a received signal to compensate for channel damage.
This method assumes that the channel is not changed in an OFDM symbol period, so damage caused by a channel change in the symbol period cannot be compensated.
When the DP-CO-OFDM channel and other channels are transmitted together in form of Wavelength Division Multiplexing (WDM), due to the nonlinear effect of an optical fiber, the other channels will exert a cross phase modulation (XPM) and a cross polarization modulation (XPoIM) on the DP-CO-OFDM signal. The XPM produces an additional phase modulation on the DP-CO-OFDM signal of the channel, and the XPoIM causes crosstalk between two orthogonal polarization states. The two nonlinear effects are both time variant, and can be deemed as a multiplicative damage and described as follows:rh(t)=whh(t)sh(t)+wvh(t)sv(t)rv(t)=whv(t)sh(t)+wvv(t)sv(t)  (1)
where, sh(t) and sv(t) are the complex amplitudes of the signal transmitted in two orthogonal polarization states, rh(t) and rv(t) are the complex amplitudes of the received signal, whh(t), wvh(t), whv(t) and wvv(t) are four time variant complex functions, which describe the XPM and XPoIM effects. The time constants of the two nonlinear effects are related to a bandwidth of the signal of an adjacent channel and a chromatic dispersion of an optical fiber link. When the signal of the adjacent channel is also a high-speed optical signal, the XPM and XPoIM effects are both fast time variant, and the time constants probably may be less than the OFDM symbol period. When the time constants are less than the OFDM symbol period, whh(t), wvh(t), whv(t) and wvv(t) cannot be regarded as constants in one symbol period, which is equivalent to a multiplicative damage of the fast time-variant channel. The traditional DP-CO-OFDM is helpless to the damage.
To be noted, the above descriptions of the conventional technology shall not be construed as well known to those skilled in the art just because they are given herein.